1. Technical Field to Which the Invention Pertains
The present invention relates to a driving force distribution device for a vehicle comprising magnetic flux density detecting means for detecting the magnetic flux density which flows in electromagnetic clutches of the vehicle, current detecting means for detecting the excitation current which flows in the electromagnetic clutches, and control means for determining the target engagement forces of the electromagnetic clutches and controlling the engagement forces of the electromagnetic clutches, wherein distribution of the driving force is carried out between driving wheels of the vehicle by means of the electromagnetic clutches.
2. The Relevant Art
There is a known technique for enhancing turning performance of vehicles where the engine driving force can be distributed between the right and left driving wheels via a driving force distribution device comprising two clutches, and the driving force distributed to the outer turning wheel is increased while the driving force distributed to the inner turning wheel is decreased so as to generate a yaw moment in the turning direction. With regard to such a driving force distribution device, one in which the above-mentioned two clutches are electromagnetic clutches has already been proposed by the present inventors (see Japanese Patent Application No. 11-176651).
In the above-mentioned electromagnetic clutches of the art, a core housing a coil and an armature are placed on either side in the axial direction of frictional engagement members, and an outer guide and an inner guide for supporting the frictional engagement members in a slidable manner are placed outside and inside the frictional engagement members in the radial direction. The above-mentioned core, outer guide, armature and inner guide form a closed magnetic circuit, and a magnetic flux generated along the above-mentioned magnetic circuit by excitation of the coil attracts the armature so as to engage the frictional engagement members.
In the above-mentioned device of the art, in order to match the driving force distributions (that is to say, the engagement forces of the electromagnetic clutches) of the driving force distribution device which have been determined from the driving conditions of the vehicle such as the engine torque, engine rotational rate, vehicle speed and steering angle with the target values, the target excitation current of the above-mentioned electromagnetic clutch corresponding to the engagement force of the electromagnetic clutch is determined and feedback control is carried out so as to match the actual excitation current of the electromagnetic clutch with the above-mentioned target excitation current.
However, when the frictional engagement members of the electromagnetic clutch are worn by long-term use, so decreasing the air gap of the magnetic circuit, it causes a problem that even when the same level of excitation current is applied to the electromagnetic clutch, the magnetic flux density so generated increases, thus increasing the engagement force of the electromagnetic clutch. It is therefore necessary in the art to provide a magnetic flux density detecting means for detecting the magnetic flux density generated in the electromagnetic clutch, and also to carry out magnetic flux density feedback control so as to match the actual magnetic flux density so detected with a target magnetic flux density. Since it is necessary for the above-mentioned magnetic flux density detecting means to be of a high-precision type which can detect dynamic changes in the magnetic flux density, there is the problem that the cost rises.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to accurately control the engagement forces of the electromagnetic clutches which govern the distribution of the driving force without carrying out magnetic flux density feedback control requiring a high-precision magnetic flux density detecting means.
In order to achieve the above-mentioned object, in accordance with the invention, there is provided a driving force distribution device for a vehicle comprising: magnetic flux density detecting means for detecting a magnetic flux density which flows in electromagnetic clutches, current detecting means for detecting an excitation current which flows in the electromagnetic clutches, and control means for determining target engagement forces of the electromagnetic clutches and controlling the engagement forces of the electromagnetic clutches, distribution of the driving force being carried out between driving wheels of the vehicle by means of the electromagnetic clutches, wherein the control means determines a relationship between the excitation current and the magnetic flux density based on the magnetic flux detected by the magnetic flux density detecting means when a predetermined excitation current is applied to the electromagnetic clutches which are not controlled for driving force distribution, converts a target magnetic flux density corresponding to the target engagement force into a target excitation current based on the relationship between the excitation current and the magnetic flux density when the electromagnetic clutches are controlled, and carries out current feedback control so that the excitation current flowing in the electromagnetic clutches detected by the current detecting means converges to the target excitation current.
In accordance with the above-mentioned arrangement, since the relationship between the excitation current and the magnetic flux density is determined from the magnetic flux density detected by the magnetic flux density detecting means while applying a predetermined excitation current when the electromagnetic clutches are not controlled, a target magnetic flux density corresponding to the target engagement force can be converted into a target excitation current using the relationship when the electromagnetic clutches are controlled for driving force distribution. Therefore, even when the frictional engagement members of the electromagnetic clutches are worn over time, so decreasing the air gaps, the target excitation current can be determined precisely from the target magnetic flux densities at that time, and the magnetic flux density feedback control which has been needed in the art is eliminated so simplifying the control system. Moreover, since there is no need for the above-mentioned magnetic flux density detecting means to detect dynamic changes in the magnetic flux density, inexpensive devices such as, for example, search coils can be used instead so contributing to a reduction in the cost.
Search coils 34R, 34L in the embodiments correspond to the magnetic flux density detecting means of the present invention, the electronic control unit U in the embodiment corresponds to the control means of the present invention, and the front right wheel WFR and front left wheel WFL in the embodiment correspond to the driving wheels of the present invention.